Fetch requirements using surface renewal analysis for estimating scalar surface fluxes from measurements in the inertial sublayer

The Surface Renewal, SR, method to estimate scalar surface fluxes is based on the scalar conservation equation and requires the measurement of the scalar (at high frequency) taken at one height as input. The distance downwind from the leading edge needed to deploy instrumentation depends on the measurements required as input in the method used for scalar flux estimation. However, a footprint model for the SR method has not been yet studied. On the basis of the Horst and Weill's (1994) Eulerian analytical flux footprint model, an equation was derived that relates the ramp dimensions required in the SR method as input with the cross-wind integrated scalar concentration distribution. It was tested using measurements of water vapor and carbon dioxide concentrations at a height of 2 m over rangeland grass (0.25 m tall). The agreement was excellent. As a consequence, the footprint model for the SR method operating in the inertial sublayer is identical to the flux-gradient, FG, method. The footprints for the eddy covariance, EC, method and for FG method are close. Consequently, for low vegetation when the eddy covariance, EC, and the SR methods operate at the same level in the inertial sublayer, the footprints may be considered identical. The SR method does not require a three dimensional sonic anemometer. Therefore, for low vegetation the SR method has the potential to be less stringent to fetch requirements than the EC method because the instrumentation required can be deployed closer to the ground.

► Knowledge of flux footprints for estimating turbulent surface fluxes is crucial.
► The footprint model for the Surface Renewal method is unknown.
► It has been studied and the advantage of the Surface Renewal method versus the eddy covariance method is discussed in relation to instrumentation required.